I don’t start with the question “100 or 200 Ah?”. I start with how much the boat consumes in a 24-hour period. Once that figure is known, the battery choice becomes much more logical.
1. Calculate consumption in Wh
For each appliance, multiply the watts by the number of hours per day. Then add all the appliances together. A 40 W appliance used for 5 hours contributes 200 Wh.
2. Convert to Ah, if that helps you
As an estimate, Wh can be divided by the system voltage. 600 Wh in a nominal 12 V system is approximately 50 Ah. I still prefer to keep track of Wh because it makes 12, 24, and 48 V systems easier to compare.
3. Choose how many days you want to be able to manage
If the boat consumes 600 Wh per day and you want to last two days without charging, you need approximately 1,200 Wh of usable energy plus an appropriate reserve.
4. Usable capacity is not the same as the nameplate
How much of the battery's nominal capacity you should plan to use depends on the battery type, BMS, and the manufacturer's specifications. Therefore, I do not use one fixed percentage for all lithium, AGM, and GEL batteries.
5. Large loads also require enough current
A battery can have many Ah and still be wrong for a large inverter. Check the battery's and BMS's permitted continuous current, fuse, cable, and the inverter's load – especially with motors, compressors, and other appliances with high start-up current.
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